Progress in miniaturization and in the capability of electronic image display devices is a key enabling technology for compact and high performance near-eye displays (NEDs) which have been extensively researched in recent years in view of increasingly popular augmented reality (AR) applications.
A challenge in the design of NEDs for AR applications has been to make such a system truly “wearable”. For this purpose, the NED system must present high quality virtual images, be comfortable to wear, and not look awkward in appearance. Further, the system must not impair the user's perception of real-world views and be sufficiently rugged for daily handling. All these requirements must be met under the constraints of low cost in mass production for wide adoption.
There are a number of prior art NEDs which attempt to meet the above requirements with varying degrees of success. See, for example, U.S. Pat. Nos. 5,696,521, 5,886,822 and 8,508,851B2 whose exemplary NEDs are shown in FIGS. 1-3 respectively. These prior art NEDs take advantage of a refractive and/or reflective system with rotationally-symmetrical optical surfaces which are simple to fabricate and have average image performance but are undesirably bulky and/or have an awkward appearance. To better fit near-eye displays into a smaller volume on a user's face, some prior art NEDs employ an off-axis optical architecture which is symmetrical but only about the horizontal plane. See, for example, U.S. Pat. Nos. 6,353,503B1, 6,710,902B2, 6,147,807 and 9,239,453B2 whose exemplary NEDs are shown in FIGS. 4-5 and 7-8. In these prior art NEDs, a tilted off-axis segment of a partially-reflective concave surface in front of a users' eye is used to image light from the display device and combine it with light from the real-world view. The “combiner” surface may be of a complex geometry such as toroidal or free-formed. It may also be a flat surface with micro-features. To balance optical aberrations that are introduced by such combiner elements, additional or other tilted/decentered and/or free-form optical surfaces are employed elsewhere in the system.
The resulting near-eye off-axis near-eye display system is undesirably difficult to fabricate and looks very different from ordinary eyeglasses and is often too bulky, thus not aesthetically pleasing from a consumer standpoint. For example, U.S. Pat. No. 9,134,535B2 discloses an exemplary NED as illustrated in FIG. 6 in which the virtual image is combined with the real-world view through an ophthalmic lens-shaped combiner. The combiner has many minute facet features on the curved front and back surfaces. Since these minute facets are made parallel to each over the curved surfaces in the transversal zone, no additional aberration is introduced as imaging light is “Total Internal Reflection”- or “TIR”-bounced over these facets before being ejected toward the eye. As a result, other imaging surfaces anterior to the combiner can have rotational symmetry and fewer optical aberrations. Unfortunately, these imaging surfaces are large in diameter due to the required increased optical path to the eye pupil as the light must be guided through multiple (TIR) bounces within the combiner. This in turn detracts from the appearance experience of the NED. Further, the discontinuity between the minute facets can cause artifacts in the information light and the real-world view. Also, the parallelism between the minute facets must typically be held to very tight tolerances due to the necessary multiple TIR bounces off of the facets.
Another challenge in the design of prior art near-eye display systems has been the need to increase the system's field of view (FOV) so more information can be presented to the user for an improved user experience.
Optically, the FOV of a near-eye display can be increased merely by increasing the image display device size while keeping the system effective focal length (EFL) and numerical aperture (NA) unchanged. The product of NA and EFL defines the radius of the NED exit pupil within which an unobstructed virtual image can be viewed. This approach leads to a more complex optical system due to the associated size increase of optical surfaces/components and the need for aberration control for an increased FOV. In spite of the trade-off between system complexity and virtual image resolution at the corner fields, the resulting near-eye display will still grow in size due to the larger image display device and larger optical surfaces/components that must be employed to avoid vignetting. This in turn makes the near-eye display less wearable due to its excessive bulkiness. A larger display device is also undesirably less power efficient and is more costly.
To increase the FOV of a near-eye display, another approach has been to divide the FOV into multiple zones and to cover each zone with a channel which uses a smaller image display device. This angular tiling method has the advantage of each channel being relatively compact and the growth of system volume is proportional to the number of channels and the extent of the FOV. Also, the use of a number of smaller display devices is more cost-effective than the use of a single large device. See, for example, U.S. Pat. No. 6,529,331B2, and U.S. Pat. No. 9,244,277B2.
FIG. 9 shows a prior art NED incorporating multiple channels used to expand the FOV as disclosed in U.S. Pat. No. 6,529,331B2. Each channel comprises a display device and covers a limited FOV which is stacked or tiled to cover a larger FOV. The exit pupil planes of the various channels are overlapped at a single point which is coincident with the eye front point. The eye clearance (“ec”) distance from the exit pupil plane to the first optical surface of a channel is defined as the eye clearance of a channel while the ec' distance from the eye front plane to the nearest optical surface among all channels is defined as the eye clearance of the NED. FIG. 9 shows that ec must be greater than ec' due to the arc arrangement of all channels around the eye front point. The associated increased ec requires that each channel grow in size and thus the entire system becomes bulkier. To fit multiple channels together, complex boundary conditions must be addressed and met which in turn increases NED fabrication challenges. To avoid a visual perception of gaps in the tiled-up image, the eye front must be finely aligned with the common point of the exit pupil planes of all channels. This requirement often makes the near-eye display difficult to wear. Further, the location of the display devices is in front along the vision line and is often not easy to use in connection with an AR application.
FIG. 10 shows a prior art NED with multiple channels to expand a FOV as disclosed in U.S. Pat. No. 9,244,277B2. A prism with a free-form optical surface is employed in each channel to deliver light from a display device image to the eye pupil. The exit pupil plane of each channel is made to be coincident with the eye pupil plane. This feature avoids the reduction of ec′ from that of ec for a single channel and makes the whole system generally more compact than that of U.S. Pat. No. 6,529,331 B2. The location of the display device is also moved away from the forward vision line for the convenience of an AR application. To undo the distortion of the real-world view caused by the imaging prism, a matched free-form prism is added anterior. To combine various channels, the imaging prism tip of each channel is trimmed and then butted together with glue or is bonded in a molding operation. Due to the required inclusion of the matched prism, this form of prior art NED typically becomes bulkier. Further, there is typically a seam line where the different channels adjoin. These seam lines often create distortion in the real-world view of the NED. Also, the trimming operation often leads to field-dependent pupil vignetting in each channel with more vignetting for the central FOV than for the peripheral FOV. The vignetted pupils of various channels complement each other at the eye pupil plane due to the symmetrical arrangement of various channels. Such a complement in pupil vignetting typically minimizes the appearance of image gaps in the tiled image as the eye rotates but to work well, the eye must be closely aligned to maintain the symmetrical arrangement of various channels. This again typically leads to a requirement of tight eye alignment with the NED.
To increase the FOV of a NED, yet another prior art approach has been to divide the FOV into a number of zones and to cover each zone in a time sequence with light from a single display device. A smaller display device can be used in this approach which in turn typically makes the NED smaller. The imaged light from the display device is switched among different zones using switchable optics components. See, for example, U.S. Pub. No. US2015/0125109A1 and U.S. Pub. No. US2014/0232651A1.
FIGS. 11A and 11B show two switchable light paths in the prior art NED disclosed in U.S. Patent Pub. No. US2015/0125109A1 with the FOV tiled in time sequence based on a multi-layer planar waveguide with a switchable Bragg Diffraction Grating (SBDG). Light from the display device is collimated by optics disposed outside the layered waveguides before being coupled into the waveguide using an input diffraction grating. In the waveguide, the display light is coupled by TR reflection and routed along various paths using grating pairs formed by non-output grating layers and output grating layers. In this prior art device, either the non-output or the output grating layer in each pair must be a SBDG. Each distinct path corresponds to a FOV tiling channel. The non-output grating layers may be input grating layers or light path folding grating layers. One of the advantages of such a system is the realization of a real-world view through a thin planar waveguide which acts as a flat eyeglass lens. Another advantage of this approach is the expansion of the eye box through multiple bounces off of the folding or output grating layers with a diffraction efficiency less than one (1). With the eye box expansion taken care of by the waveguide and embedded grating layers, the collimation optics anterior to the waveguide can have a relatively small exit pupil aperture and simpler architecture which helps the performance of the NED. The small exit pupil aperture of collimation optics make the collimation optics smaller, but it typically also matches the very small geometries of the waveguide. However, disadvantages of such a system often include: a) low optical efficiency due to the small aperture of the collimation optics anterior to the waveguide and the switching in time sequence; b) the high complexity of the SBDG system, its environmental susceptibility (temperature/pressure/stress) and endurance; c) grating-induced optical aberrations for which correction complicates the collimation optics architecture or lowers the system optical performance; d) restrictions in the achievable FOV imposed by the TR and grating angular bandwidth; e) a large area on the planar waveguide is needed to accommodate various grating pairs to route the display light and to expand the eye box size; and; f) the near-eye display volume is increased by the required collimation optics attached to the waveguide.
FIG. 12 shows yet a further prior art NED with multiple channels used to expand the FOV as disclosed in U.S. Patent Pub. No. US2014/0232651A1. The system is based on time multiplexing with active components such as switchable mirrors or holographic optics. The display device and imaging optics are disposed in a straight line to simplify optics and reduce the aberration level. For the real-world view light path, the near-eye display acts like an optical plate and introduces no distortion. Since the FOV is stacked up with spatially-separated folding components, the optical path length for each channel varies greatly. This leads to the field-dependent vignetting at the eye pupil which undesirably varies from channel-to-channel. The near-eye display as a result has a relatively small eye box.
Another significant technical hurdle to the miniaturization of near-eye display systems is the availability of high brightness and compact display devices, as can be inferred from the above discussion. Common conventional display device technologies include Digital Micro-Mirror Display (DMD), Liquid Crystal Display (LCD), Liquid Crystal on Silicon (LCOS) and Organic Light Emitting Diode (OLED). Systems such as DMD and LCOS require an accompanying illumination optical system which adds system volume. LCD technology has associated low brightness and lower resolution. OLED technology is more compact than DMD and LCOS and has better brightness and resolution than LCD. OLED is also a promising display device format for near-eye displays, but OLED still needs to further improve its brightness and durability for wide adoption in NED applications.
A new class of emissive micro-scale pixel array imager devices has been introduced as disclosed in U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,567,960, and 8,098,265, the contents of each of which is fully incorporated herein by reference. The disclosed light emitting structures and devices referred to herein may be based on the Quantum Photonic Imager or “QPI®” imager. QPI® is a registered trademark of Ostendo Technologies, Inc. These disclosed devices desirably feature high brightness, very fast multi-color light intensity and spatial modulation capabilities, all in a very small single device size that includes all necessary image processing drive circuitry. The solid-state light—(SSL) emitting pixels of the disclosed devices may be either a light emitting diode (LED) or laser diode (LD), or both, whose on-off state is controlled by drive circuitry contained within a CMOS chip (or device) upon which the emissive micro-scale pixel array of the imager is bonded and electronically coupled. The size of the pixels comprising the disclosed emissive arrays of such imager devices is typically in the range of approximately 5-20 microns with a typical emissive surface area being in the range of approximately 15-150 square millimeters. The pixels within the above emissive micro-scale pixel array devices are individually addressable spatially, chromatically and temporally, typically through the drive circuitry of its CMOS chip. The brightness of the light generated by such imager devices can reach multiple 100,000 cd/m2 at reasonably low power consumption.
The QPI imager referred to in the exemplary embodiments described below is well-suited for use in the wearable devices described herein. See U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,567,960, and 8,098,265. However, it is to be understood that the QPI imagers are merely examples of the types of devices that may be used in the present disclosure. Thus, in the description to follow, references to the QPI imager, display, display device or imager are to be understood to be for purposes of specificity in the embodiments disclosed, and not for any limitation of the present disclosure.